Some air-conditioning apparatuses such as multi-air-conditioning apparatuses for building are configured to circulate refrigerant, for example between an outdoor unit installed outdoors and indoor units located inside the rooms, to perform a cooling operation or heating operation. More specifically, the refrigerant transfers heat to air so as to heat the air or removes heat from the air so as to cool the air, and such heated or cooled air is utilized to heat or cool the space to be air-conditioned. In such a type of air-conditioning apparatus, for example hydrofluorocarbon (HFC)-based refrigerant is often employed. In addition, air-conditioning apparatuses that employ a natural refrigerant such as carbon dioxide (CO2) have also been proposed.
Further, air-conditioning apparatuses differently configured, typically represented by a chiller system, have been developed. In this type of air-conditioning apparatus, cooling energy or heating energy is generated in the outdoor unit installed outdoors, and a heat medium such as water or antifreeze solution is heated or cooled with a heat exchanger provided in the outdoor unit. Then the heat medium is transported to the indoor unit located in the region to be air-conditioned, such as a fan coil unit or a panel heater, so as to cool or heat the region to be air-conditioned (see, for example, Patent Literature 1).
In addition, an outdoor-side heat exchanger, called exhaust heat collection chiller, is known in which the outdoor unit and the indoor units are connected via four water pipes, and cooled or heated water is supplied at the same time so as to allow each of the indoor units to select cooling or heating operation as desired (see, for example, Patent Literature 2).
Further, an air-conditioning apparatus is known in which a heat exchanger for heat exchange between the refrigerant and the heat medium is located in the vicinity of each indoor unit, and the heat medium is supplied from the heat exchanger to the indoor unit (see, for example, Patent Literature 3).
Further, an air-conditioning apparatus is known in which the outdoor unit and branch units each including a heat exchanger are connected via two pipes, so as to supply the heat medium to the indoor unit (see, for example, Patent Literature 4).
Still further, an air-conditioning apparatus is known in which the outdoor unit and a relay unit are connected via two refrigerant pipes, and the relay unit and the indoor units are connected via two pipes through which a heat medium such as water circulates, so as to transfer heat from the refrigerant to the heat medium in the relay unit, thereby allowing the cooling and heating operation to be performed at the same time (see, for example, Patent Literature 5).